The high performance of filed emission from well-aligned carbon nanotubes has been achieved by optimizing the thickness of an MgO coating layer on the nanotubes. It has been clarified that an MgO layer as thin as 2 nm degrades the emission properties and a thicker MgO layer does not improve the properties effectively. The most stable emission as low as 9% for 8 hrs with the lowest turn-on voltage of ∼116 V is obtained for the thickness of 10 nm. The 10 nm-MgO coated nanotubes also exhibit a uniform emission from the whole area of nanotubes. The theoretical calculation of the potential distribution around the emission sites supports the existence of this optimum thickness.